Update "JWT security" section

[begriffs]

https://postgrest.com/en/v4.4/auth.html#jwt-security

It still talks as if asymmetric keys are not supported.

[steve-chavez]

Paragraph that should be corrected:

Another more subtle attack is possible where servers use asymmetric algorithms like RSA for signatures. Once again this is not relevant to PostgREST since it is not supported.